High silica based optical fibres are firmly established as the most efficient interconnection media for optical telecommunication networks. The fibres are used as the passive transmission media to guide optical signals over long distances. In contrast, rare-earth (RE) ions if doped into the core of such fibres, make them optically active due to the characteristic emission of the RE when pumped at suitable wavelengths. Because of this property RE doped fibres have shown great potential for use as active devices for photonic applications like optical amplifiers and fibre lasers at various wavelengths. The fibres are also found to be promising candidates for their application as sensors for monitoring temperature, radiation dose etc.
Erbium doped fiber which is the active medium of an EDFA (erbium doped fiber amplifier) has been an enabling technology for optical networks operating in the third telecommunication window between 1530 and 1610 nm. EDFA can simultaneously amplify several optical channels in a single fibre which has enabled the implementation of DWDM (dense wavelength division multiplexing) technology with the potential of increasing the bandwidth of long distance transmission systems from Gb/s to Tb/s ranges. EDFAs exhibit high gain, large bandwidth, low noise, polarisation insensitive gain, substantially reduced cross talk problems and low insertion losses at the operating wavelengths. The deployment of EDFA has spurred a tremendous growth in advanced telecommunication systems replacing the conventional optoelectronic repeaters. While the Erbium Doped Fibre (EDF) remains the most important for telecommunication applications, fibres doped with other rare earths are gaining importance mostly for development of laser sources from visible to mid infrared regions. Development of broadband amplifiers commencing from 1300 nm is an area of great interest using various REs. Lasing and amplification have been demonstrated at several wavelengths with the incorporation of the various rare-earths.
Reference may be made to Townsend J. E., Poole S. B., and Payne D. N., Electronics Letters, Vol. 23 (1987) p-329, ‘Solution-doping technique for fabrication of rare-earth-doped optical fibre’ wherein the Modified Chemical Vapour Deposition (MCVD) process is used to fabricate the preform with a step index profile and desired core-clad structure while solution doping is adopted for incorporation of the active ion. The steps involed in the process are as follows:                A conventional cladding doped with P2O5 and F is deposited within a high silica glass substrate tube to develop matched clad or depressed clad type structure.        The core layers of predetermined composition containing index raising dopant like GeO2 are deposited at a lower temperature to form unsintered porous soot.        The tube with the deposit is immersed into an aqueous solution of the dopant precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the dopant ion is suitable for preparation of the solution although rare earth halides have been mostly used.        Following immersion, the tube is rinsed with acetone and remounted on lathe.        The core layer containing the RE is dehydrated and sintered to produce a clear glassy layer. Dehydration is carried out a temperature of 600° C. by using chlorine. The level of OH− is reduced below 1 ppm using Cl2/O2 in the ratio of 5:2, provided the drying time exceeds 30 min.        Collapsing in the usual manner to produce a solid glass rod called preform.        Fibre drawing is conventional.        
Reference may also be made to DiGiovanni D. J., SPIE Vol. 1373 (1990) p-2 “Fabrication of rare-earth-doped optical fibre’ wherein the substrate tube with the porous core layer is soaked in an aqueous or alcoholic solution containing a nitrate or chloride of the desired RE ion. The tube is drained, dried and remounted on lathe. The dehydration is carried out by flowing dry chlorine through the tube at about 900° C. for an hour. After dehydration, the layer is sintered and the tube is collapsed to be drawn to fibre.
Reference may also be made to Ainslie B. J., Craig S. P., Davey S. T., and Wakefield B., Material Letters, Vol. 6, (1988) p-139, “The fabrication, assessment and optical properties of high-concentration Nd3+ and Er3+ doped silica based fibres” wherein optical fibres based on Al2O3—P2O5.—SiO2 host glass doped with high concentrations of Nd3+ and Er3+ have been fabricated by solution method and quantified. Following the deposition of cladding layers P2O5 doped silica soot is deposited at lower temperature. The prepared tubes are soaked in an alcoholic solution of 1M Al(NO3)3+various concentrations of ErCl3 and NdCl3 for 1 hour. The tubes are subsequently blown dry and collapsed to make preforms in the usual way. Aluminium (Al) is said to be a key component in producing high RE concentrations in the core centre without clustering effect. It is further disclosed that Al and RE profile lock together in some way which retards the volatility of RE ion. The dip at the core centre is observed both for P and GeO2.
Reference may also be made to U.S. Pat. No. 5,005,175 (1991) by Desuvire et al., ‘Erbium doped fiber amplifier” wherein the fibre for the optical amplifier comprises a single mode fibre doped with erbium in the core having a distribution profile of the RE ion whose radius is less than 1.9 μm while the radius of the mode of the pump signal exceeds 3 μm. The numerical aperture (NA) of the fibres varies from 0.2 to 0.35 and the core is doped with both Al and Ge oxides to increase the efficiency. The fibre with such design is reported to have increased gain and lower threshold compared to the conventional Er doped fibre amplifiers.
Reference may also be made to U.S. Pat. No. 5,778,129 (1998) by Shukunami et. al., ‘Doped optical fibre having core and clad structure for increasing the amplification band of an optical amplifier using the optical fibre’ wherein the porous core layer is deposited after developing the cladding inside a quartz tube by MCVD process and solution doping method is employed to impregnate Er as the active ion into the porous core to be followed by vitrification and collapsing for making the preform. The solution also contain compounds of Al, say chlorides, for co-doping of the core with Al in order to expand the amplification band. The Er and Al doped glass constitutes first region of the core. Surrounding this are the second and third regions of the core. The third region contains Ge to increase the refractive index. The second region has an impurity concentration lower than both those of first and third regions and consequently low RI also. The second region acts as a barrier to prevent diffusion of the active dopant.
Reference may also be made to U.S. Pat. No. 5,474,588 (1995) by Tanaka, D. et. al., ‘Solution doping of a silica with erbium, aluminium and phosphorus to form an optical fiber’ wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited on a seed rod (VAD apparatus) to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an aluminium compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The desiccation is carried out for a period of 24-240 hours at a temperature of 60°-70° C. in an atmosphere of nitrogen gas or inert gas. This desiccated soot preform is heated and dehydrated for a period of 2.5-3.5 hours at a temperature of 950°-1050° C. in an atmosphere of helium gas containing 0.25 to 0.35% chlorine gas and further heated for a period of 3-5 hours at a temperature of 1400°-1600° C. to render it transparent, thereby forming an erbium doped glass preform. The segregation of AlCl3 in the preform formation process is suppressed due to the presence of phosphorus and as a result, the doping concentration of Al ions can be set to a high level (>3 wt %). The dopant concentration and component ratio of Er, Al and P ions are claimed to be extremely accurate and homogeneous in radial as well as in longitudinal directions.
Reference may also be made to U.S. Pat. No. 5,123,940 (1992) by DiGiovanni et. al., ‘Sol-Gel doping of optical fibre preform’ wherein the method comprises collapsing a silica—based glass tube to make a preform and drawing fibre from the preform. Before collapsing the tube, one or more glass layers are formed in the inner surface of the glass tube by dip-coating with a stable dispersion (sol) containing the metal-alkoxides and dopant cations including RE-ions. The metal-alkoxides dissolved in an alcoholic or aquous solvent contains required quantity of the dopants, polymerising the sol to form a gel, drying and sintering the tube. A wide variety of dopant materials, in the form of salts or alkoxides are easily incorporated by dissolving them in the solvent. The method suffers disadvantage that there is a possibility of evaporation of the RE-salts during sintering at high temperature, there by causing inhomogeneous distribution of RE-ions through out the length of the preform.
Reference may also be made to the publication by Matejec et al., ‘Properties of optical fibre preforms prepared by inner coating of substrates tubes’ Ceramics-Silicaty, 45 (2), 62 (2001) wherein the method consists of collapsing a silica based glass tube containing the required dopant cations to a preform and drawing fibre from the preform. Before collapsing the tube, one or more glass layers are formed in the inner surface of the glass tube by raising and lowering the sol level at a fixed velocity. The sol contained the silicon tetraethoxide (TEOS) and dopant cations including RE-ions. The TEOS dissolved in an alcoholic or aqueous solvent contains required quantity of the dopants, polymerizing the sol to form a gel followed by drying and sintering the tube. The main disadvantage of the method is that there is every possibility of evaporation of the RE salts during sintering at a high temperature, resulting in an inhomogeneous distribution of RE-ions throughout the length of the preform.
A few of the Drawbacks of the Above Mentioned Processes are as Follows:                1. Deposition of porous silica soot layer at a temperature of 1200-1400° C. by Chemical Vapour Deposition (CVD) process inside a substrate silica tube or on a seed rod (VAD or OVD apparatus).        2. The porosity of the soot layer controls the RE incorporation and the homogeneity along the length of the preform.        3. The control of porosity of the deposited unsintered layer is difficult as it is extremely sensitive to the deposition temperature, burner traverse speed and flow of the reactant materials. This leads to variation in soot density and composition along the length of the tube.        4. The dipping procedure of the soot containing preform into the RE solution is critical due to the possibility of generation of local imperfections and concentration variation in the core of the preform.        5. Minor variation in dipping parameters coupled with the porosity of the unsintered soot layer leads to considerable change in RE concentration as well as radial distribution in the core of the preform.        6. The dipping parameters and the porosity of the soot layer are critical to obtain good core-clad interface and minimise attenuation of the fibre.        7. The dopant materials are concentrated in the pores of the deposited layer. As a result, clusters or microcryastallites of dopants tend to form both before sintering and during the steps of sintering and collapsing of the glass materials, giving rise to inhomogenuous distribution of the dopant materials.        8. Formaton of microcrystallites causes scattering of light and increases the attenuation of the fibre.        9. Evaporation of the solvent leaves behind a residue containing the salt of the dopant cations or RE oxychloride which during dehydration in the chlorine atmosphere or sintering at high temperature volatilises, creating a dip in concentration near the inner surface of the porous layer.        10. Dehydration and sintering of the RE chloride impregnated soot layer give rise to compositional variation due to the vaporization of the dopant salt as well as GeO2 in the core.        11. The process suffers from reliability/repeatability due to its sensitivity to the parameters during various stages of processing such as deposition, solution doping, drying and sintering.        